Stabilization of synthetic elastomers



Patented Mar. 28, 1950 OFFICE STABILIZATION F SYNTHETIC ELASTOMERS Ernst P. Bittershausen, Hempstead, and Paul D.

Sharpe, New York, N. Y.,

Incorporated, a corpo- Vacuum Oil Company,

ration of New York assignors to Socony- No Drawing. Application March 23, 1948, Serial No. 16,634

1 The present invention has to do with stabilization of synthetic elastomers and, more particularly, has to do with the stabilization of butadlene-styrene elastomers and elastomeric compositions containing butadiene-styrene elastomers.

As is well known in the art. elastomeric com- 8 Claims. (0]. 260-453) positions are somewhat unstable, particularly when in contact with oxidizing materials. For many years, therefore, natural rubber has been protected against oxidation by various materials which have been designated antioxidants." With the development of a variety of synthetic rubbers or elastomers, there has again arisen the problem of preventing or retarding oxidation. Many of the antioxidants effective when used in small amounts of natural rubber. have been found to be efl'ective also when so used with one or more synthetic elastomers. In a number of instances, however, antioxidants for natural rubber have proven to be inefiective when used with a synthetic elastomer. In a similar manner certain antioxidants having a high degree of effectiveness in protecting a particular synthetic elastomer have been of little value when used wherein:

with natural rubber. Accordingly, it is well recognized that a catalytic action is involved in inhibiting oxidation of natural and synthetic elastomers.

It has now been found that certain polymeric materials, defined hereinafter, are particularly effective antioxidants when used in small amounts with butadiene-styrene copolymers known in the art as "GR-S elastomers.

Butadiene-styrene copolymers are synthetic elastomers developed during the past decade, and have been identified in the art bv the t rm GR-S. They are described in "Specifications for Government Synthetic Rubbers, issued by Ofllce of Rubber Reserve, and in numerous publicatl'ons. As shown in the aforesaid "S ecifications, GR-S elastomers are com ri ed of approximately parts by weight of butadiene and 25 parts by weight of styrene. 'I'he'copolvmerization reaction is conducted by the emulsion polymerization process. In this process, monomers of butadiene and styrene are individually emulsifled in water with the aid of soaps or similar materials. The monomer emulsions are combined and a polymerization catalyst such as potassium persulfate is added. Polymerization is conducted in a closed vessel for approximately 15 ho rs at about F. To obtain a copolymer of desired properties. the nolvmerizatinn react on is stopped by the addition of a material such as hydroquinone, The unreacted monomers are removed by distillation, and the resulting latex is coagulated with an acid or salt solution. The preci itated curds of elastomer are then washed and dried.

In general, all GR-S elastomers are effectively stabilized by the polymeric materials contemplated herein. Particularly preferred, however. are GR-S materials of the character described on page 2 of the aforesaid Specifications. Such materials are free of foreign or extraneous material objectionable in normal rubber practice, and have had incorporated during manufacture approximately 1.25% of a standard rubber antioxidant.

The polymeric materials referred to above are obtained by reacting thiophene, or substitutedthiophenes having stable electroposltive substituents other than unstable hydroxyl (0H) and amino (N112) groups attached to the thiophene nucleus, with formaldehyde or its polymers, and with an ammonium halide or an amine salt. The preparation of these polymeric materials is described in co-pending application Serial No. 636,511, filed December 21, 1945, of Howard D. Hartough and Sigmund J. Lukasiewicz. As set forth in said co-pending application, the polymeric materials are amines and appear to be represented by the following general formula:

[(W) a(Y) 11(2) e(OH) alMHX) 0 W is one of the group, thiophene and thiophenes having stable electropositive groups attached to the nucleus;

Y is a methylene group:

Z is nitrogen;

(OH) is part of the molecule reacting with acetic anhydride as in the ASTM method for deterthe molar ratios, of the reactants .results in a variation in molecular weight of the polymeric amines. By proper selection of proportions, polymeric amines having molecular weights of the order of 500 to 1500 and greater can be obtained. Of such polymeric amines, we have found that those having molecular weights from about 850 to about 1500 (110%) are effective stabilizers for "GR-S" elastomers. Particularly preferred of the efiective polymeric amines, however, are those having a molecular weight of about 1000 to about 1250 (110%) and the following characteristics: Sulfur content, from about 22 to about 25; Nitrogen content, from about 3 to about 6; Hydroxyl number (ASTM), from about 200 to about 350.

The aforesaid effective polymeric amines are prepared by reaction 01' thiophene or substituted thiophene, formaldehydeor polymer thereof, and an ammonium halide, all as described hereinabove. The particularly preferred polymeric amines are prepared by reacting the aforesaid reactants in a ratio of 1:2:1.

As indicated hereinabove, the molecular weights are subject to experimental error of 4 Physical:

Viscosity of "GR-S 46 (min.)

ML 212 F. at 4 mins-.54 (max.) Viscosity of compounded stock:

ML 212 F. at 4 mins '13 (max.) Properties of vulcanizate at 82 F.:

about per cent; an experimental error long 10 Tensile strength, 50 recognized in the art. The hydroxyl number, min. cure at 292 referred to above, is determined by the ASTM F .2700 p. s. i. (min.) method, and is indicative of the part of the Ultimate elongation, molecule of the polymer reacting with acetic an- 50 min. cure at hydride. Under the conditions of the ASTM test, 292 F .550% (min.) groups other than the hydroxyl group react with Modulus at 300% elonacetic anhydride; a typical group is the imino gation: group. min. cure at 292 For a more complete understanding of the F {350 p. s. l. (min.) polymeric amines contemplated herein, the folg 650 p. s. 1. (max.) lowing illustrative example is set forth below. 50 min. cure at 292 This example is shown as Example VII of the F {800 p. s. i. (min.) copending application identified above, and the 1200 p. s. i. (max) .polymeric amine obtained is the amine of said 90 min. cure at 292 Example VII. Reference is made to said co- :5 F {1150 p. s. l. (min.) pending application for further description and 1550 p. s. i. (max.) representative examples. The antioxidant or stabilizing characteristics EXAMPIE of the polymeric amines described above are illustrated by the following test data with a typical A mixture of 168 grams (2 mols) of thloph basic gum stock formulation. The antioxidant 120 grams (4 mob) of parafolmaldehyde, 103 properties of the polymeric amine obtained in rams (2 mols) of a o Chloride and 20 the example above are compared with those of grams of acetic acid were heated for 1 hours phenyl beta-napht,hylamine i t following at 74 0. The reac o product thus obtained formulation (Table I). As is well known in the was treated with a saturated aqueous solution t pheny1 beta-naphthy1amine is an efl ti e of 200 grams of crystalline sodium acetate, and rubber antioxidant 300 milliliters of benzene. A liquid was separated Table I from the resulting mixture. This liquid is pre- Parts by weight dominantly comprised of the desired amine. It 100 100 was treated with caustic solution and the amine Stearic acid 1 1 freed therefrom was dissolved in benzene. Ben- Zinc oxide n 10 1o zene was distilled off and the amine was analyzed. Mercapto benzothiazole u 1 1 The amine had the following properties: Sulfur 3 3 Sulfur, 23.1 per cent; nitrogen, 4.67 per cent; Phenyl-beta-naphthylamine 1 hydroxy number, 257; molecular weight, 1123. Polymeric amine ple I) 1 l The GR-S" material used in obtaining data 1 0.86 weight per cent. in Tables I and H, shown below, is of the character described on page 2 of the aforesaid Speciii fg gfiggg f g zs gao g fi ggf' fications. The following are specification limits tiveness of the polymeric thiophen amine for the materialthe superiority thereof over phenyl-beta-naph- Chemical: thylamine, are shown by tensile strength data in Volatile, matter 0.50% (max) Table II below. The change in tensile strength Ash 1.50% (maX.) of standard dumbbell test strips is determined by E-T-A extract .10.00% (max) '15 measuring the tensile strength before and after Fatty acid (as stearic artificial aging, which simulates conditions noracid) 3.75% (min.) mally encountered. Strips of the cured (or comf 6.00% (max) pounded) materials are artificially aged by ex- Soap (soluble as sodium posing the same for 24 hours in a forced draft stearate) 0.75% (max) oven maintained at 121 C.

Table II Before Aging Alter Aging Per Cent strips wlth Antoxidam ($1 53. Per Cent Tensile Per Cent Tensile Chan? Elonga- Strength Elonga- Strength Tenslle tion (p. s. i.) tion (p. s. i

Phenyl-beta naphthylamine 30 820 1, 020 140 1, 180 -28 45 500 1,000 150 1,320 -18 00 380 1,860 150 1.560 -1e 250 1, 340 120 1, 320 -2 Polymeric Amine (Example I) 30 280 2,230 L330 40 45 200 1,880 100 1, 480 -22 00 190 1,830 100 1,870 +2 90 200 1,860 1,800 0 As those skilled in the art know, there-is an optimum time of cure for each rubber or elastomer formulation, the optimum being based upon balanceof elongation and tensile strength characteristics. The data in Table II, above, indicate that the optimum cure is between 45 and 60 minutes at 274 F. for the formulation containing a typical antioxidant, phenyl-beta-naphthylamine; whereas, the optimum cure period for the formulation containing the polymeric amine of Example I is about 60 minutes at 274 F. The data demonstrate that, at optimum cure, the latter formulation has practically the same tensile strength after aging as before aging. 'This is in contrast with the substantial loss in tensile strength, at optimum cure, of the formulation containing phenyl-beta-naphthylamine.

Elastomeric formulations, or basic gum stocks,

as illustrated by those shown in Table I, above,

generally contain other materials in addition to the elastomer and antioxidant. Typical of such materials are the following: lubricant, such as stearic acid and other fatty acids, soaps of fatty acids such as zinc stearate; an inorganic accelerator-activator such as zinc oxide, oxides of lead or magnesium, lead salts, lime, etc.; accelerators such as mercaptobenzothiazole, mercaptans, disulfides, etc.; vulcanizing agents such as sulfur, selenium or tellurium, or organic compositions containing one or more of said elements and from which is liberated one or more of said elements during curing. These materials are used in proportions such as shown in the formulations shown in Table I, above, and as usually found in "GR- -containing formulations.

The effective polymeric amines provide satisfactory results when used within the range of about 075 part by weight to about 2.00 parts by. weight, for 100 parts by weight of GR-S" elastomer.

It is to be understood that the invention is not limited by the foregoing description and illustrative examples, but is to be broadly construed in the light of the language of the appended claims.

We claim: 1. In a basic gum stock containing a butadiene-styrene elastomer, the improvement which comprises: incorporating therein a small amount,

sufllcient to stabilize said stock against oxidation, of a polymeric thiophene amine having a mgolecular weight between about 850 and about 1 00.

2. A basic gum stock containing a butadienestyrene elastomer and a small amount, suflicient to stabilize said elastomer against oxidation, of a polymeric thiophene amine having a molecular weight between about 850 and about 1500.

8. A stable elastomeric composition comprising a butadiene-styrene elastomer and a small amount of a polymeric thiophene amine having a molecular weight between about 850 and about 6 4 1500, from about 0.75 part by weight to about 2.00 parts by weight of said amine being present with 100 parts by weight of said elastomer.

4. A basic gum stock-consisting essentially of:

I said polymeric thiophene amine containing at least one thiophene nucleus per polymer unit, having a molecular weight of about 1123, an hydroxyl number (ASI'M) of 257, and containing about 23.1 per cent sulfur and about 4.67 per cent nitrogen.

5. The method of stabilizing a basic gum stock containingabutadiene-styrene elastomer, which comprises:' incorporating in said stock a small amount, sufficient to stabilize said stock, of a polymeric thiophene amine having a molecular Weight of about 850 to about 1500.

6. In a basic gum stock containing a butadienestyrene elastomer comprising approximately parts by' weight of butadiene and 25 parts by weight of styrene, the improvement which comprises: incorporating therein a small amount, from about 0.75 part by weight to about 2.00 parts by weight for parts .by weight of said elastomer, of a polymeric thiophene amine having a molecular weight between about 1,000 and about 7. In a basic gum stock containing a butadienestyrene elastomer comprising approximately 75 parts by weight of butadiene and 25 parts by weight of styrene, the improvement which comi. prises: incorporating therein a small amount,

from about 0.75 part by weight to about 200 parts by weight for 100 parts by weight of said elastomer, of a polymeric thiophene amine containing at least one thiophene nucleus per polymer unit, having a molecular weight of about 1123, an hydroxyl. number (ASTM) of 257, and containing about 23.1 per cent sulfur and'about 4.67 per cent nitrogen.

8. The method of stabilizing a basic gum stock containing a butadiene-styrene elastomer comprising approximately 75 parts by weight of butadiene and 25 parts by weight of styrene, which comprises: incorporating in said stock a small amount, from about 0.75 part by weight to about 2.00 parts by weight for 100 parts of said elastomer, of a polymeric thiophene amine having a molecular weight between about 1000 and about ERNST P. RITTERSHAUSEN. PAUL D. SHARPE.

No references cited. 

1. IN A BASIC GUM STOCK CONTAINING A BUTADIENE-STYRENE ELASTOMER, THE IMPROVEMENT WHICH COMPRISES: INCORPORATING THEREIN A SMALL AMOUNT, SUFFICIENT TO STABILIZE SAID STOCK AGAINST OXIDATION OF A POLYMERIC THIOPHENE AMINE HAVING A MOLECULAR WEIGHT BETWEEN ABOUT 850 AND ABOUT
 1500. 